title: Human thymic epithelium and T cell development: current issues and future directions.
abstract: The human thymus develops early in fetal gestation with morphologic maturity reached by the beginning of the second trimester. TE3+ cortical thymic epithelium is most likely derived from endodermal third pharyngeal pouch, while A2B5/TE4+ medullary and subcapsular cortical thymic epithelium is likely derived from third pharyngeal cleft ectoderm. Fetal liver and yolk sac CD7+, CD4-, CD8-, surface(s) CD3- T cell precursors begin to colonize the thymus between 7 and 8 weeks of fetal gestation, followed by rapid expression of other T lineage surface molecules on developing thymocytes. CD7+, CD4-, CD8-, sCD3- thymocytes give rise to T cells of both the TCR alpha beta and TCR gamma delta lineages. Human thymic epithelial cells produce numerous cytokines including IL1, IL6, TGF alpha, leukemia inhibitory factor (LIF), M-CSF, G-CSF and GM-CSF- molecules that likely play important roles in multiple stages of thymocyte selection, activation and differentiation. Important areas for future research on human thymic epithelium include study of lymphoid and non-lineage differentiation potentials of CD7+, CD4-, CD8-, sCD3- T cell precursors in response to TE-cell produced cytokines, study of the triggering signals of cytokine release within the thymic microenvironment, and study of TCR-MHC mediated TE-thymocyte interactions.
authors.affiliation: Department of Medicine, Duke University Medical Center, Durham, NC 27710.
authors.forename: B F
authors.initials: BF
authors.lastname: Haynes
mesh_terms: D002454:Cell Differentiation; D004847:Epithelial Cells; D006801:Humans; D013601:T-Lymphocytes / Q000166:cytology* / Q000276:immunology; D013950:Thymus Gland / Q000166:cytology* / Q000196:embryology / Q000276:immunology